The present invention relates generally to the sizing and drawing of synthetic textile filaments and, in particular, to such methods and apparatus wherein the filament is conveyed successively through a sizing bath, a driven filament squeezing mechanism or device, a driven drying roller spaced downstream from the squeezing mechanism to define a draw zone therebetween and driven at a differential relative speed to the squeezing mechanism for draw stretching of the filament while traveling through the draw zone, where the filament will be simultaneously pre-dried, and a winding machine or device for final take-up of the drawn filament.
West German Offenlegungsschrift DE-OS 36 02 968 A1 discloses a filament sizing and drawing system of the aforedescribed type whose preferred embodiment is adapted for simultaneous operation on a plurality of textile filaments traveling in side-by-side relation to one another in the form of a warp sheet. In the described apparatus and method, draw stretching of the multiple filaments is accomplished in the area between the squeezing mechanism and the succeeding drying cylinder or roller wherein the filaments remain wetted with sizing, which serves to assure a defined filament temperature during stretching and further eliminates the need for any additional machine units to accomplish drawing of the filament sheet. Within the draw and pre-drying zone, the individual filaments of the filament sheet exhibit a defined temperature, namely, the essentially constant dew point temperature which automatically develops in the pre-drying zone after the filaments leave the sizing bath, generally in the range of approximately 60.degree. C.
As is known, in order for the draw stretching to which the filaments are subjected in the draw zone to be effective for increasing alignment and ordering of the molecular and crystalline structure of the filaments, the filaments being drawn must be at a temperature exceeding the glass transition temperature for the particular filamentary material Since polyamide filaments may be effectively drawn at temperatures in the range of the aforesaid dew point temperature typically prevailing in the filaments as they pass through the squeezing mechanism, the above-described known method and apparatus has proven satisfactory for drawing of polyamide filaments. However, on the other hand, this known method and apparatus is generally ineffective for drawing of other textile filaments, particularly partially oriented and low oriented polyester filaments, commonly referred to as POY and LOY filaments, since polyester filaments can only be effectively drawn at considerably higher temperatures in the range, for example, of 80.degree. to 100.degree. C. Depending upon the particular filamentary material, e.g., polyamide or polyester, and its initial state, e.g., POY or LOY, filaments may be drawn to a degree or ratio which may range, for example, between 1.3 and 3.2 times the starting length of the filament. Thus, it will be understood that such drawing of synthetic filaments involves a relatively significant permanent lengthening of the filament while at a temperature elevated above the filament's glass transition temperature, as opposed to a temporary, essentially elastic elongation of filaments occurring below the applicable transition temperature.
Within the framework of the method and apparatus disclosed in the aforesaid West German Offenlegungsschrift DE-0S 36 02 968 A1, it would be impractical and counterproductive to elevate the filament temperature in the area of the squeezing mechanism sufficiently to accommodate drawing of POY and LOY-type polyester yarns because the elevated temperature and the correspondingly lengthened processing time to which the filaments would be subjected to heating would cause the sizing to prematurely dry on the filaments and prevent the sizing from elongating to the same degree as the filaments, thereby causing a tendency of the dried sizing to crack. Moreover, since the known method and apparatus relies largely upon the essentially constant dew point temperature which naturally occurs in the filaments upon leaving the squeezing mechanism, this known method and apparatus would suggest that additional heating of the filaments should be avoided.